Startup system

ABSTRACT

An improved startup system for a vapor generator of the oncethrough type. The system includes a startup bypass line which leads directly to the tube side of a shell and tube feedwater heater for the generator feedwater circuit, from certain heating passes of the generator main flow circuit; a branch line which leads from the bypass line to the shell side of the heater; and a return line which returns the flow from the heater shell side back to the generator main flow circuit downstream of the bypass line. A pressure reducing valve in the branch line reduces the pressure of the fluid flowing therein lowering its temperature for heat exchange with fluid on the tube side of the heater. The feedwater circuit of the generator includes a bypass around the heater, the generator further having shutoff valves in both the startup system and feedwater circuit for isolating the heater from one or the other of the main flow or feedwater circuits depending upon use of the heater.

United States Patent [72] Inventor William D. Stevens Primary Examiner- Kenneth W. Sprague North Caldwell, NJ. Attorneys-John Maier, Ill, Marvin A. Naigur and John E. (2| I Appl. No. 842,283 Wilson [22] Filed July [6, 1969 [45] Patented July 6, I971 [73] Auisme m m ABSTRACT: An improved startup system for a vapor genera- LM-mn, tor of the once-through type. The system includes a startup bypass line which leads directly to the tube side of a shell and tube feedwater heater for the generator feedwater circuit, from certain heating passes of the generator main flow circuit; [54] SYSTEM a branch line which leads from the bypass line to the shell side 5 CHI, 1 Dn'h I of the heater; and a return [me which returns the flow from the heater shell side back to the generator main flow circuit US. downeu-eam of the line A pressure redu ing valve in cl F215 35/1 the branch line reduces the pressure of the fluid flowing 0' Search............................................ therein lowering temperature for heal exchange 406 S on the tube side of the heater.

The feedwater circuit of the generator includes a bypass (56] CM around the heater, the generator further having shutoff valves UNITED STATES PATENTS in both the startup system and feedwater circuit for isolating 3,l83,896 5/ I965 Lytle et a] 122/406 the heater from one or the other of the main flow or feedwater 3,3,237 4/ 1967 Strohrneyer, Jr. 60/105 circuits depending upon use of the heater.

r24 74 26 |2\ PR! FIN SH SH w t rl- 56 58 l4 :6 J l gm 38 I 2:: "40 l I! if '54 m ECON 20 b 34 L y 1 i i I u 76 "HF u F a F 72 4e 42 1o 1 4 l lf l t i H I l8 L M PATENTED JUL 6197i INVENTOR.

WILL/AM D. STEVENS Nb VI? I if t w H J I! G Mm a Q. 8 vw 3v! Nm 9? L zoom 2E MP Em All STARTUP SYSTEM DESCRIPTION The present invention relates to vapor generators of the once-through type, and in particular to an improvement in the startup system for a once-through vapor generator.

A typical once-through vapor generator, of the type to which the present invention pertains, includes a main flow heating circuit connected to a suitable point of use, such as a circuit on the other hand, usually includes a deaerator, lowand high-pressure feedwater heaters, and a feedwater pump means between the heaters.

During startup of the vapor generator, the low enthalpy fluid cannot be handled by the high-pressure turbine, or certain superheating surfaces, and for this reason, the generator is usually provided with a bypass system to handle the flow until the flow is at the enthalpy level required. It is known to posi tion in the bypass line a pressure reducing valve which adiabatically lowers the pressure and temperature of the fluid to produce a vapor-liquid mixture. This mixture is transmitted to a flash tank or separator which separates the flow into vapor and liquid streams, the vapor stream then being transmitted back to the main flow path for early warming of the downstream heating surfaces or passes and early warming and rolling of the high-pressure turbine. The vapor also is required for such uses as turbine gland sealing, the pegging the deaera- Of.

US. Pat. No. 3,314,237, applied for by Charles Strohmeyer, describes a startup system in which a heat exchanger is interposed in the bypass line, the heat exchanger being of the shell and tube type. The bypass line is connected to the heat exchanger so that the high-pressure side thereof receives the high-pressure flow directly from the upstream heat absorption passes of the generator. A branch conduit from the bypass line transmits a portion of the high-pressure flow through a pres sure-reducing valve and then to the shell or low-pressure sidb of the heat exchanger. The throttled portion of the flow is hdt only reduced in pressure but also in temperature, so that in the heat exchanger, the high-pressure high-temperature portion of the flow gives up heat to the lower temperature lower pressure flow. The latter after heat exchange is returned to the generator downstream heat absorption passes for such uses as warm ing the passes, and warming and rolling the turbine.

It is described in the patent that because of the heat exchanger, the firing rate increase in the generator can be made linear with respect to the increase in steam flow in the furnace circuits of the generator up to the point where the bypass system is isolated from the flow and the flow is transmitted directly from the upstream absorption passes to these downstream of the bypass system. This is made possible by coordinating with the increased firing and flow rates an increase of flow through the branch conduit of the bypass system, that is the throttled flow to be heated on the shell side of the heat exchanger. The increase in flow through the branch conduit is accompanied by an increase in downstream pressure as the throttling valve is opened, so that eventually, conditions in the downstream heat absorption passes substantially match those upstream of the bypass system.

One problem experienced with the bypass system of prior U.S. Pat. No. 3,3l4,237, similar to the problem experienced with conventional systems utilizing flash tanks or separators, is that a special vessel is required for the system. Vapor generators are becoming larger in size and capacity, and the bypass systems of necessity are required to handle ever increasing quantities of flow. The vessels must have heavy walls designed to withstand high pressures and temperatures, so that these vessels more and more are becoming major capital cost items in the generator.

It is an object of the present invention to overcome the above problem, and in particular to provide a simplified bypass system of reduced capital cost, which is capable of permitting a linear increase in firing rate with the increase in flow through the high-pressure absorption passes of the generator.

It is further an object of the invention to provide a bypass system in which the need for a special vessel to handle the flow in the system is eliminated, and which instead utilizes in this respect already existing equipment in the generator.

in accordance with the present invention, there is provided a startup system including a startup bypass line which leads from certain heating passes of the generator main flow circuit directly to the tube side of a shell and tube feedwater heater of the generator feedwater circuit. A branch line leads from the bypass line to the shell side of the feedwater heater, and a return line returns the flow from the heater shell side back to the generator main flow passes downstream of the bypass line. A pressure-reducing valve in the branch line reduces the pressure of the fluid flowing therein lowering its temperature for heat exchange with fluid on the tube side of the heater.

The feedwater circuit of the generator includes a bypass around the heater, the generator further comprising shutoff valves in both the startup system and feedwater circuits for isolating the heater from one or the other of the main flow circuit or feedwater circuit depending upon use of the heater.

The invention, objects and other advantages thereof will become more apparent on consideration of the following specification, with reference to the accompanying drawing, in which:

The FIGURE is a schematic drawing of a vapor generator flow circuit and bypass system in accordance with the present invention.

Referring to the drawing, the vapor generator includes a flow path generally indicated with the number 12 com prising a main flow heating circuit 14 leading to a point of use 16, which may be a conventional high-pressure turbine. A feedwater heating circuit 18 is provided to supply a fluid flow to the main flow circuit. Now shown are, additional turbines, a condensing means between the turbines and feedwater circuit for receiving the exhaust flow from the turbines, drains in the main steam line, drains required during initial heating of the finishing superheater, and turbines stop valves.

The main flow heating circuit comprises an economizer 20, radiant heating furnace passes 22, a primary superheater 24, and a finishing superheater 26, the latter being connected to the high-pressure turbine 16. As will be shown, the economizer, furnace passes, and primary superheater constitute during startup the high-pressure portion of the main flow circuit, and the finishing superheater surface or pass is usually held at a lower pressure. On course, during normal operation of the generator, the entire main flow path up to the turbine is at a high pressure.

The feedwater heating circuit 18 consists of a deaerator 28 leading to a feedwater pump 30, the flow from the pump being to first, second and third high-pressure heaters 32, 34, and 36 respectively. The heaters are conventionally arranged in series with exhaust flow from the shell side of the third heater cascading to the second, and so forth. The system may include additional low-pressure heaters upstream of the feedwater pump 30, and other variations are possible.

Also shown in the drawing is a startup bypass system generally indicated with the numeral 38 connected to the main flow path between the primary superheater 24 and finishing superheater 26.

in accordance with the present invention, the bypass system comprises a high-pressure line 40 leading from the outlet end of the primary superheater 24 directly to a high-pressure inlet header 42 of the third feedwater heater 36. The latter is of the shell and tube type having a shell 44 encompassing a tube bundle 46, the bundle being U-shaped extending between the inlet high-pressure header 42 and an outlet header 48. That flow which is transmitted to the inlet header 42 passes through the tubes of the heater to the outlet header 48.

Also constituting part of the bypass system is a branch line 50 teed to the high-pressure line 40, the branch line leading to the shell side of the feedwater heater. A pressure reducing valve 52 in the branch line reduces the pressure of the flow. The feedwater heater is conventionally baffled on its shell side so that the flow travels a generally U-shaped path in a direction opposite to the direction of flow in the tubes, the exhaust flow from the shell side of the heater being transmitted via return line 54 to the main flow heating circuit 14 and particularly to the inlet end of the finishing superheater 26.

in the high-pressure line 40 and return line 54 are shutoff valves 56 and 58 which isolate the bypass system 28 from the main flow circuit.

Also provided in accordance with the present invention is a conduit 60 to receive the high-pressure bypass flow from the heater outlet header 48. This flow is reduced in pressure by a pressure reducing valve 62 in the conduit and is transmitted to the shell side of the second feedwater heater 34 for heating the feedwater flow in the feedwater circuit 18 in a conventional manner. A pressure controller 64 responsive to the pressure in the shell of the second heater reduces the pressure of the cue haust bypass flow to that which can be accommodated by the shell.

Further provided in accordance with the present invention is a bypass conduit 66 connected in the feedwater circuit to bypass the heater 36. The bypass conduit leads from the inlet side of the heater to its downstream side or to the inlet of the economizer 20. Positioned in the bypass conduit is a shutoff valve 68 which permits the conduit to be used alternately with the heater in the feedwater circuit, the latter including shutoff valves 70 and 72 on opposite sides of the heater. The latter two valves are between the heater and bypass conduit connections to isolate the heater from the feedwater circuit when the bypass system is being used.

In operation, a flow of water is established in the generator by starting the feedwater pump 30, the initial flow being through the two feedwater heaters 32 and 34, through the bypass conduit 66, and into the main flow heating circuit M. The shutoff valves 70 and 72 are closed to isolate the feedwater heater 36 from the feedwater circuit. A shutoff valve 74 between the points of connection of the high-pressure line 40 of the bypass system and the return line 54 also is closed so that the flow from the primary superheater 24 is into the highpressure line 40. At this stage, throttling valve 52 in the branch line 50 is closed, so that all of the flow into the line 40 is through the tube side of the heater 36 to conduit 60 and the second heater 34 respectively, the flow being then cascaded down to heater 32 and recycled to the deaerator 28.

The burners (not shown) for the generator are fired increasing the fluid temperature and enthalpy of the flow through the main flow circuit l4. At first, the increased enthalpy flow cascaded downwardly through the heaters imparts heat to the feedwater primarily in the deaerator.

As the temperature of the flow in the bypass continues to rise, the pressure in the bypass and also in the heating circuit [4 including the primary superheater, furnace passes and economizer, is controlled by pressure reducing valve 62 at the inlet to the shell side of heater 34. It is controlled by a preset limit of pressure controller 64 responsive to pressure within the shell of the heater, for instance, at about 3,550 p.s.i.

It is possible that not all of the bypass flow would be utilized by the heaters 32, 34 for feedwater heating, and if desired, excess flow can be diverted elsewhere, for instance, to the condenser, or the deaerator 30, in a known manner.

When sufficient heat is available in the bypass flow for heating the finishing superheater surface, or heating and rolling the turbine 16, the throttling valve 52 is partially opened to admit flow to the shell side of heater 36, which flow is transmitted via return line 54 to the finishing superheater and turbine. The pressure reduction in the throttling valve 52 is ac' companied by a substantial temperature reduction. Thus, the temperature of the high-pressure fluid passing through the multiple tubes of the heater is substantially higher than the temperature of the low-pressure fluid discharging from valve 52. A heat transfer occurs between the high-pressure fluid and low-pressure fluid with the latter receiving heat, so that the heat content of the fluid in return line 54 is thereby at a substantially higher level than that of the flow from the primary superheater into the bypass system. Ideally, the pressure of the flow through valve 52 is reduced to about L000 p.s.i., held by controller 76 responsive to pressure in the heater shell, this pressure and the temperature of the flow in the return line 54 being suitable for warming the finishing superheater 26, and early warming and rolling of the turbine.

As heating is continued in the steam generator and further steam is made available, ultimately the turbine is rolled, then synchronized and loaded to a predetermined minimum value by further opening valve 52. Then firing of the burners is increased along with flow rate in the generator, the rate of increase in firing being made linear to the rate of increase in flow. During opening of valve 52, there is a pressure and temperature increase of the flow into the finishing superheater and turbine. Ultimately the enthalpy of the flow in line 54 is such that it substantially matches that in line 40. At this point, the shutoff valve 74 in the main flow path can be opened and when fully open, shutoff valves 56 and 58 can be closed isolating the bypass system from the main flow heating circuit. Also, at this point, shutoff valves 70 and 72 in the feedwater circuit are opened, and shutoff valve 68 in the bypass conduit 66 is closed, for flow of feedwater through the heater 36 in a normal manner. From here on, and during normal operation of the generator, the heater 36 is used for feedwater heating.

Advantages of the invention should now be apparent. For one the invention takes advantage of the fact that particularly in the large vapor generators of today, there are multiple feedwater heaters, and the full capacity of the heaters is not required during the startup period. In addition, the invention takes advantage of the fact that the heat transfer surface of a conventional feedwater heater makes it ideally suitable for the present application. In particular the amount of heat transfer surface is such, in relation to the temperature differential of the two heat exchange fluids in question that even though the flow in line 40 is substantially below saturation, the flow through valve 52 receives sufficient heat to be heated to reasonable dryness. This means that after a minimum startup period the enthalpy in line 54 can be made to substantially match that in line 40, without other than a linear increase in firing rate with increase in flow rate into the generator.

Of course, a paramount advantage is that a special heat exchanger or flash vessel is not required.

Although the invention has been described with respect to a specific embodiment, variations within the scope of the following claims will be apparent to those skilled in the art.

lclaim:

1. in a startup system for a once-through vapor generator of the type including a main flow path having heating surfaces therein, a feedwater circuit for said main flow path, said circuit including a plurality of feedwater heaters, at least one of which is of the tube and shell type, the improvement comprising a bypass line connected between said main flow path and the tube side of said tube and shell feedwater heater; a branch conduit in flow communication with said main flow path leading to the shell side of said tube and shell feedwater heater; a pressure reducing means in said branch line; a return line leading from said shell side back to said main flow path downstream of said bypass line; a bypass conduit in said feedwater circuit around said tube and shell heater; and means to isolate said tube and shell heater from either the main flow path or the flow in the feedwater circuit, the tube and shell heater thereby being employed either as a feedwater heater or a heat exchanger for the generator bypass flow.

2. The system of claim I including shutoff valves to isolate the tube and shell heater from the feedwater circuit, further including a shutoff valve in the bypass conduit and shutoff valves between said tube and shell heater and the main flow path to isolate the heater from the main flow path.

3. The system of claim 1 wherein said pressure-reducing means includes a controller responsive to pressure in the shell of the tube and shell heater.

4. The system of claim 1 including at least two heaters, said shell and tube heater being downstream of the other heater in the feedwater circuit, further including a conduit connected to 6 receive the exhaust flow from the tube siae of the shell and 5. The system of claim 4 including pressure-reducing means tube heater and to transmit the flow to said other heater for in said conduit.

heat exchange with the feedwater flow therein. 

1. In a startup system for a once-through vapor generator of the type including a main flow path having heating surfaces therein, a feedwater circuit for said main flow path, said circuit including a plurality of feedwater heaters, at least one of which is of the tube and shell type, the improvement comprising a bypass line connected between said main flow path and the tube side of said tube and shell feedwater heater; a branch conduit in flow communication with said main flow path leading to the shell side of said tube and shell feedwater heater; a pressure reducing means in said branch line; a return line leading from said shell side back to said main flow path downstream of said bypass line; a bypass conduit in said feedwater circuit around said tube and shell heater; and means to isolate said tube and shell heater from either the main flow path or the flow in the feedwater circuit, the tube and shell heater thereby being employed either as a feedwater heater or a heat exchanger for the generator bypass flow.
 2. The system of claim 1 including shutoff valves to isolate the tube and shell heater from the feedwater circuit, further including a shutoff valve in the bypass conduit and shutoff valves between said tube and shell heater and the main flow path to isolate the heater from the main flow path.
 3. The system of claim 1 wherein said pressure-reducing means includes a controller responsive to pressure in the shell of the tube and shell heater.
 4. The system of claim 1 including at least two heaters, said shell and tube heater being downstream of the other heater in the feedwater circuit, further including a conduit connected to receive the exhaust flow from the tube side of the shell and tube heater and to transmit the flow to said other heater for heat exchange with the feedwater flow therein.
 5. The system of claim 4 including pressure-reducing means in said conduit. 